Motion compensation (MC) is known as a means for ensuring high coding efficiency. Several widely known video coding algorithms such as CCITT H.261 ("Codec for Audiovisual Services at n.times.384 kbit/s," Rec. H.261, pgs. 120-128, 1988), ISO/MPEG DIS 11172 ("Coding for Moving Pictures and Associated Audio," ISO/IEC JTC 1/SC 29N 071 pgs. 2-A-18, Dec. 6, 1991) and other international standard coding methods employ a hybrid coding system which combines motion-compensating interframe prediction and discrete cosine transform (DCT). Under this strategy, the incoming signal is sub-divided into blocks of N.times.N samples called macroblocks and each block is processed independently. FIGS. 1A and 1B depict a hybrid motion-compensated interframe/intraframe DCT (MC/DCT) coder from the prior art.
MC/DCT coders are efficient and achieve high compression ratios, but they also result in blocking effects, especially at low coding rates, primarily because each macroblock is coded independently of other neighboring macroblocks. These blocking effects are perceived as visible discontinuities in the image at boundaries and result in an overall reduction in image quality.
These discontinuities are a result of two factors. First, at low bit rates, the DCT coefficients are quantized coarsely, thus resulting in intensity level differences at block boundaries. Second, under conventional MC techniques, motion vectors are generated by the block matching method, in which a typical vector is selected for each 16-picture-element.times.16-line block. For this reason, if motion vectors vary between neighboring blocks, discontinuities in the levels of the predicted signal and prediction error signal are apparent at boundaries between the neighboring macroblocks.
In recent years, the Lapped Orthogonal Transform (LOT) has been proposed as another transform useful for video coding (The LOT: Transform Coding without Blocking Effects, H. S. Malvar and D. H. Staelin, IEEE Trans. ASSP. Vol. ASSP-37, no. 4 pp. 553-559, April 1989; Lapped Transform for Efficient Transform/Subband Coding, H. S. Malvar, IEEE Trans. Acoustics, Speech, and Signal Processing, Vol. ASSP-38, no. 6, pp. 969-978, June 1990; and Analysis/Synthesis Filter Bank Design Based on Time Domain Aliasing Cancellation, J. P. Princen and A. B. Bradley, IEEE Transactions on Acoustics, Speech and Signal Processing, Vol. ASSP-34, No. 5, pp. 1153-1161, October 1986). The LOT is an attractive transform in that it employs a lapped block structure wherein a frame of video is segmented into blocks which overlap. This overlapping block structure prevents block distortion and improves coding efficiency. Unlike the DCT, the LOT basis in a given block overlaps neighboring blocks which causes redundant information to be transmitted for samples at the block boundaries and reduces blocking effects without an increase in the bit rate under some schemes.
In particular, LOT is an attractive alternative to DCT since it minimizes blocking effects prevalent in DCT. However, employing LOT, in lieu of DCT, with MC has not resulted in significant improvements in coding efficiency over MC/DCT since some of the advantages of LOT are lost due to blocking effects resulting from the block matching method employed in MC. To reduce the effects of blocking artifacts at boundaries in MC/LOT coders, LOT coefficients must be quantized finely, which results in a reduction in the overall coding efficiency.
Windowed Motion Compensation (WMC) has been found useful in reducing blocking effects when used with the DCT in lieu of conventional motion compensation. WMC is a technique to reduce discontinuities in predicted images perceived at the block boundaries due to MC. WMC reduces the prediction error in the image at MC block boundaries, thus increasing the coding efficiency. WMC, unlike MC, assumes that the predicted image is formed by a superposition of windowed (and possibly shifted) blocks from the previous image. However, WMC is computationally expensive and difficult to use with LOT because of the mismatch between the LOT basis and the window function used in WMC.
An object of our invention is a coder and coding method which achieves the high coding efficiency of motion compensation without blocking effects at block boundaries prevalent in conventional motion compensation coders due to level differences in the prediction error signals and predictive signals between neighboring blocks. Also, an object of our invention is to minimize blocking effects resulting from DCT while achieving the high coding efficiency characteristic of analysis/synthesis filter banks such as LOT.